Need some help with a gas flow problem

I have a steel box that I am trying to purge with gaseous nitrogen (GN2). Just say the dimensions are A x B x C (ft^3). I would like to maintain a continuous purge at a very minimal pressure (~0.5 inH20). To conserve money (i.e. so that I don't need to purchase an expensive flow regulation device), I am simply looking to tie into an already-existent GN2 supply line nearby (to get my flow in) and then add a vent line to get my flow out. The net flow should maintain a very low positive pressure inside the panel. The supply line is pressurized to 100 psi, so I'd need to add an orifice to prevent overpressure. My question is: how can I size my vent and orifice to ensure that my box remains at my requirements (positive low pressure purge)? Do I need a low pressure check valve? Will the outlet diameter or the inlet diameter need to be larger (or does it not matter)?